A mask is a bitmap that you apply to the baseline and result bitmaps in order to exclude any part of a bitmap from comparison by the Bitmap Tool. For example, if you are testing a custom object that is painted on the screen and one part of the object is variable, you might want to create a mask to filter out the variable part from the bitmap comparison.
You might consider masking any differences that you decide are insignificant or that you know will vary in an effort to avoid test case failure. For example, suppose a test case fails because one bitmap includes a flashing area of a dialog box. In the Bitmap Tool you can block the flashing area from the two bitmaps by creating and applying a mask to them. Once a mask is applied and the masked bitmaps are saved, the mask becomes a permanent part of the baseline bitmaps you are comparing. Masks can also be saved in separate files and used in test cases.
The Open Agent and Classic Agent capture bitmaps in different color depths. By default, the Open Agent captures bitmaps using the current desktop settings for color depth. The Classic Agent captures bitmaps as 24-bit images. If you create a mask file for the captured bitmap using the Bitmap tool, the mask file is saved as a 24-bit bitmap. If the bitmap mask file does not have the same color depth as the bitmap that you capture, an error occurs. To ensure that VerifyBitmap functions do not fail due to different color-depth settings between the captured image and the mask image, ensure that the bitmaps have the same color depth.